In a fiber laser, light propagating through a cladding (unnecessary light) may heat a coating which covers the cladding. To remove such light propagating through a cladding, the following structures, for example, have been proposed.
Patent Document 1 discloses an optical device including a first optical fiber and a second optical fiber fusion spliced with the first optical fiber, in which the diameter of the second optical fiber is greater than that of the first optical fiber. In the vicinity of the fusion splice point between the first optical fiber and the second optical fiber, the first optical fiber is surrounded by a first medium having a lower refractive index than that of the outermost shell structure of the first optical fiber and the outermost shell structure of the second optical fiber, while at least part of the second optical fiber is surrounded by a second medium having a higher refractive index than that of the outermost shell structure of the first optical fiber and the outermost shell structure of the second optical fiber.
According to the optical device, it is possible to prevent light propagating though the outermost shell structure of the first optical fiber toward the second optical fiber from leaking out to the first medium but allow the propagating light to leak out to the second medium surrounding the second optical fiber having a greater diameter.
Patent Document 2 discloses a cladding-mode absorber unit including two end regions and a center region, the two end regions each having a smaller diameter, the center region having a greater diameter sandwiched by the two end regions and covered by an absorber made of liquid metal. Since the absorber is arranged to directly contact with the cladding of the center region, light propagating through the cladding can be absorbed by the absorber in the center region.